The norepinephrine (NE) innervation to the prefrontal cortex (RFC) is involved in complex cognitive functioning, and pathologies in this system are implicated in mental health disorders including depression, post-traumatic stress disorder, and attention deficit hyperactivity disorder. NE transmission in the RFC is regulated in part by uptake through its plasma membrane transporter (NET). In the first ultrastructural immunocytochemical study of its type, we showed that most NE terminals in the RFC of naive rats contain NET primarily within the cytoplasm (i.e. not on the membrane) and lack detectable levels of the synthetic enzyme tyrosine hydroxylase (TH). Only a minority of NE terminals express appreciable plasmalemmal NET and detectable levels of TH. The basis for the predominantly cytoplasmic localization of NET and lack of TH in the majority of RFC NE terminals is not known. However, we hypothesize that these factors are influenced by the level of activity in the NE system as well as the availability of extracellular dopamine (DA), a precursor for NE. In support of this hypothesis, we have demonstrated that exposure to chronic cold stress, a treatment that persistently activates the locus coeruleus (LC) NE system, increases both the plasmalemmal localization of NET and the expression of detectable TH within the RFC. In the proposed functional anatomical studies, we will further examine the factors that regulate the subcellular distribution of NET and expression of TH in the rat RFC using immunocytochemistry and electron microscopic analysis. In Aim 1, we will study the effects of acutely altering the activity of the LC and its RFC projections using electrical or pharmacological stimulation. In Aims 2 and 3, we will examine the impact of chronic treatment with antidepressant drugs that selectively block NET, both in naive rats (Aim 2) and in animals simultaneously exposed to chronic cold stress. Finally, given the known ability of NET to transport DA, we hypothesize that NE terminals in the RFC utilize extracellular DA to synthesize NE in the absence of TH. Hence, in Aim 4, we will examine how the availability of extracellular DA affects NET distribution and TH expression in RFC NE terminals by lesioning the DA cell bodies in the ventral tegmental area. The findings from these studies will provide valuable information regarding the normal functions of the central NE system and its role in the pathophysiology and treatment of mental disorders.